1. Field of the Invention
The present invention relates generally to panels of sandwich-type composite structure having a cellular core, in particular for motor vehicles, and more particularly to a method of making such a panel whose structure is reinforced locally.
The present invention also relates to a reinforced composite panel of the cellular-core sandwich type obtained by performing such a method.
2. Background Art
Sandwich-type materials having cellular cores have very important characteristics resulting from their being light in weight.
Conventionally, such a panel is constructed by sandwiching a cellular core having low strength characteristics by gluing it or bonding it between two skins, each of which is much thinner than the cellular core but has excellent mechanical characteristics.
The Applicant""s document FR 2 711 573 discloses a method of making a panel of sandwich-type composite structure having a cellular core. In that method, said panel is made in a single step by subjecting a stack to cold-pressing in a mold, which stack is made up of at least a first skin made of a stampable reinforced thermoplastics material, of a cellular core made of a thermoplastics material, of a second skin made of a stampable reinforced thermoplastics material, and of a first external covering layer made of a woven or non-woven material, said skins being pre-heated outside the mold to a softening temperature.
Such a method is particularly advantageous because of the fact that it makes it possible, in a single operation, both to generate cohesion between the various layers of the composite structure, and to shape said panel.
The resulting panel conserves all of the mechanical properties imparted by the cellular-core sandwich structure.
Panels of sandwich-type composite structure having a cellular core have strength characteristics sufficient to enable mechanical structures subjected to large stresses to be reinforced structurally without making them too heavy. Such panels are in common use in shipbuilding, aircraft construction, and rail vehicle construction.
However, the non-uniformness of the mechanical stresses to which they are subjected sometimes makes it necessary to form local reinforcing plies at those places in said panels where the mechanical stresses are greatest.
In the field of aircraft construction, sandwich-structure composite panels are made that are based on thermo-settable resins reinforced with glass fibers.
In order to impart the desired shapes to said panels, and to maintain said shapes, the glass fibers and the thermo-settable resin (in the form of pre-impregnates) are deposited layer-by-layer in a mold, and are then heated to high temperatures so as to cure (i.e. polymerize) the resin permanently.
The molds used may have a punch or a die, or else both a punch and a die.
Making such locally-reinforced panels consists firstly in defining zones where stresses are concentrated in the resulting panels, such zones being defined either by real testing or by computer simulation, and then in adding reinforcing plies at those places so as to make it possible to withstand such stresses.
The reinforcing plies are one-directional mats or woven fabrics of glass fibers, of carbon fibers, or of natural fibers embedded in a thermo-settable resin, with an orientation that is determined by the orientation of the stresses. They are cut out to a pattern using special machines, e.g. water-jet cutting machines.
The reinforcing plies are disposed layer-by-layer in a mold, either manually or by means of a robot, with each ply having its own orientation.
That operation may be referred to as the xe2x80x9claying upxe2x80x9d operation.
Then comes the baking step which is the longest step of the method of making such pieces because the stack of layers must be heated sufficiently to cure the thermo-settable resin.
The various layers disposed in the mold are pressed in said mold by evacuating said mold. Such evacuation serves to press the materials against the die or the punch, and to remove surplus resin.
The desired shape is thus obtained with the fibers being impregnated with the resin as well as possible.
That xe2x80x9claminationxe2x80x9d technique, and in particular the xe2x80x9claying upxe2x80x9d operation, is characterized by a very low level of automation, and a large labor input.
Although, by means of the concept of localizing the strength, that technique makes it possible to achieve performance levels that are high for the pieces that are made in that way, it requires rigorous monitoring of quality.
As a result, that technique is very costly and cannot be used at the high production throughputs implemented in the field of the automobile industry.
The present invention therefore proposes a novel method of making reinforced composite panels of the cellular-core sandwich type, which method is simple and cheap, and can be implemented at high throughputs compatible with the production throughputs of the automobile industry, while also making it possible, without giving rise to any particular extra weight or extra cost, to make pieces that are suitable for structural applications which put their strength under stress.
More particularly, the invention provides a method of making a reinforced composite panel of the sandwich type having a cellular core, said method being characterized in that:
a stack is formed that is made up of: at least one first skin made of a reinforced thermoplastics material; a first reinforcing ply made of a reinforced thermoplastics material; a cellular core made of a thermoplastics material; a second reinforcing ply made of a reinforced thermoplastics material; and a second skin made of a reinforced thermoplastics material; each of the first and second reinforcing plies having a surface area that is smaller than the surface area of each of the first and second skins, and said first and second reinforcing plies being positioned symmetrically with respect to the plane formed by the cellular core at determined places against said skins;
the stack is pre-assembled;
the pre-assembled stack is heated in an oven; and
said panel is formed by subjecting the heated stack to cold-pressing in a mold, under a pressure lying in the range 1xc3x97106 Pa to 3xc3x97106 Pa.
According to other advantageous and non-limiting characteristics of the method of the present invention:
said stack may include at least one other pair of third and fourth reinforcing plies, each of which has a surface area that is smaller than the surface area of each of the first and second skins, said third and fourth reinforcing plies being disposed symmetrically about the cellular core at determined places against said skins;
the stack may include three pairs of reinforcing plies organized in determined manner, symmetrically about the cellular core; and
said stack may also include at least one outer covering layer made of a woven or a non-woven fabric disposed on the second skin, and optionally another outer covering layer made of a woven or a non-woven fabric placed under the first skin in the stacking order.
Advantageously, the forming pressure for forming the panel lies in the range 15xc3x97105 Pa to 20xc3x97105 Pa.
While said panel is being formed, the first and second skins have a forming temperature lying in the range approximately 160xc2x0 C. to 200xc2x0 C.
The first and second skins are constituted by a woven fabric or mat of glass fibers and of a thermoplastics material.
The reinforcing plies are also constituted by a woven fabric or mat of glass fibers and of a thermoplastics material.
The first and second skins have glass fiber weight per unit area that is different from that of the reinforcing plies.
The thermoplastics material used is a polyolefin and preferably polypropylene.
Preferably, the cellular core of the panel has an open-celled structure of the tubular or honeycomb cell type, constituted mainly by polyolefin and preferably polypropylene.
The invention also provides a reinforced composite panel of the sandwich type having a cellular core, made by performing the above-mentioned method.
The invention and how it may be implemented can be well understood from the following description given with reference to the accompanying drawing which is given by way of non-limiting example, and in which: